Surgical implants made of materials absorbing into the organ system, i.e. biodegradable materials, are known. Their use in surgery is continuously increasing, because they offer a significant advantage over conventional implants made of metal: they need not be removed from the system after the operated tissue has healed. This way, it is possible to avoid surgery to remove the implant, which is naturally advantageous with respect to patient satisfaction, resource load and costs.
However, metal provides the advantage over biodegradable materials that an implant made of metal is quite well distinguishable from the tissue of the organ system. Good visual distinguishability facilitates considerably the work of the surgeon in implanting the implant in the correct position and in the correct manner into the organ system. Biodegradable materials are typically translucent and light-colored and consequently, an implant made of them is poorly distinguishable from tissue. For instance, in anterior cruciate ligament ACL or posterior cruciate ligament PCL reconstruction, either a soft tissue graft, which can typically be a hamstring graft, for instance, or a bone-tendon-bone graft is fastened to holes made at the ends of the femur or tibia. The operation is usually performed as an endoscopic operation. It is of primary importance that the surgeon easily and reliably sees the position and location of the used fixation means, such as interference screws, pins or wedges, in the object being operated. This is not always the case when using biodegradable fixation means.
Coloring agents have been routinely used in dyeing biodegradable sutures since the beginning of their use, i.e. for over 20 years. One problem with dyed biodegradable sutures is that they cannot be radiation sterilized using the conventional 60Co gamma radiation process, because the polymers used in their preparation split too much under radiation and the suture does not keep its strength long enough for the tissue to heal properly (Wound Closure Biomaterials, Ed. C. C. Chu et al. CRC Press 1997, Boca Raton, Fla., U.S.A, page 183). Thus, they need to be sterilized by methods that do not split the suture material.
Implants made of biodegradable materials are known that are dyed with a coloring agent to improve their visibility. Due to the above-mentioned problems with dyed sutures, the dyed implants are not sterilized by radiation but with ethylene oxide (EO), for instance, (Athanasiou, K. A. et al., Biomaterials 17 (1996) 93 to 102). An advantage of EO sterilization is that it does not affect the implant material or its additives, such as the color pigment. A problem with EO sterilization is a possible toxic EO residue that remains in the polymer. The EO sterilization process is, therefore, time consuming, complex and expensive. The EO residue needs to be removed from the material with an aeration period that is performed in a sterilization chamber and takes several days. In addition, the packing process of the product has two phases and is, therefore, difficult. The surgical implant must first be packed in material that allows EO gas to penetrate, for instance in a Tyvec© bag. On top of this, a packing material is required to protect the product from moisture, such as an aluminized plastic bag. Because aluminized plastic does not allow EO gas to penetrate, it can be seamed tight only after the first sterilization/aeration period. The use of an aluminum bag in an operating theater also requires that the aluminum bag be sterilized on the outside. This is why after seaming the aluminum bag, a third layer of an EO gas permeable Tyvec© bag is put on the aluminum bag, and the outside of the aluminum bag is sterilized in a second sterilization phase. All in all, EO sterilization is a complex and quite expensive sterilization method.
Even though coloring agents have long been used in suture materials, their use in implants is still rare and limited to small-sized implants sterilized with EO gas. An example of such a product is a Meniscal BioStinger® arrow dyed with Violet D&C No. 2 pigment and used in treating meniscus ruptures, which is marketed by Linvatec Corporation, Largo, Fla., U.S.A., and another example, in which D&C Violet No. 2 pigment is used, is a S-D-sorb™ E-Z Tac anchor used in treating joint capsules of the humerus, which is marketed by Surgical Dynamics Inc., Norwalk, Conn., U.S.A. The chemical name of D&C Violet No. 2 pigment is 1,4-hydroxy[(4-methylphenyl)amino]-9,10-anthracenedion (CAS No. 81-48-1). The amount of color pigment used in biodegradable sutures and small-sized implants usually varies between 0.1 and 0.3 percent by weight as calculated from the weight of the implant. As high an amount of color pigment as this, which is acceptable in small-sized implants, increases the number of tissue reactions when the operation requires one or more large implants (Pelto-Vasenius K., et al. Arch. Orthop. Trauma Surg. (1998)117: 159 to 162).